The present invention relates to a polymer electrolyte fuel cell stack that works at ordinary temperature and is used for portable power sources, electric vehicle power sources, and domestic cogeneration systems.
The polymer electrolyte fuel cell generates the electricity and the heat simultaneously by reacting a fuel such as hydrogen and an oxidant gas such as the air, electrochemically at gas diffusion electrodes with a catalyst like platinum carried thereon.
One example of the polymer electrolyte fuel cell stack is shown in the partially omitted perspective view of FIG. 4.
On the opposite faces of a polymer electrolyte membrane 3, which selectively transports hydrogen ions, catalytic reaction layers 2. which are constituted by carbon powder with a platinum metal catalyst carried thereon, are closely formed. And, according to the requirement, a fluorocarbon water repellent may be added.
The polymer electrolyte used here may be a fluorocarbon polymer with sulfonate groups introduced into the ends of their side chains. This electrolyte has proton conductivity in the wet state. In order to activate the fuel cell, it is accordingly required to keep the polymer electrolyte in the wet state. The polymer electrolyte in the wet state has strong acidity due to H30  dissociated from the sulfonate groups at the ends. Accordingly, the acid resistance is required for the material of the portions that are in direct contact with the electrolyte. The equivalent material to the electrolyte is also admixed to the reaction electrodes, so that the acid resistance is required for the material of the portions that are in direct contact with the reaction electrodes.
Further, on the respective outer faces of the catalytic reaction layers 2A, a pair of diffusion layers 1 having both the gas permeability and the electrical conductivity are closely formed. This catalytic reaction layer 2 and the diffusion layer 1 constitute an electrode (an anode or a cathode).
In the case where pure hydrogen is used as the fuel, as a material constituting the anode and cathode, the same material can be used. In the case where the fuel is a gas mainly containing hydrogen, which is obtained by reforming a hydrocarbon fuel, carbon monoxide is naturally contained in the reformed gas. In order to prevent the noble metal catalyst from being poisoned with carbon monoxide, there is a proposal to add an anti-CO poisoning substance, such as ruthenium only to the anode side.
Outsides of the electrode, conductive separator (bi-polar) plates 4 are further arranged so as to mechanically fix the assembly of these electrolyte membrane and the electrodes and connect adjoining assemblies electrically with each other in series. In a portion of the separator plate 4 that is in contact with the electrode, a gas flow path 5 is formed to feed the supply of the reaction gas to the surface of the electrode and flow out the gas evolved by the reaction and the remaining excess gas. And, the gas manifolds 8, which supply a gas to and exhaust a gas from the fuel cells, and water manifolds 14, which supply water for cooling the fuel cell stack down and exhausts the water. A cooling means such as a cooling plate may be provided to the separator plate 4 may have.
In order to prevent the hydrogen gas and the air from being leaked from the cell laminate or from being undesirably mixed with each other, an internal sealing structure is general one, in which sealing portions or O-rings are disposed around the electrodes across the polymer electrolyte membrane.
Since the above-mentioned proton-conductive electrolyte has strong acidity, a fluorocarbon polymer material having high acid resistance is employed for sealing portions like gaskets that are in direct contact with the electrolyte.
With a view to maximizing the area of the electrodes, an external sealing structure may in adopted, which does not use the sealing portions or O-rings around the electrodes but extends the ends of the electrodes to the side face of the cell laminate and seal the side face of the cell laminate with an air-tight non-conductive material.
The polymer electrolyte fuel cell stacks of the external sealing structure are divided into an internal manifold type and an external manifold type. In the internal manifold type, the manifolds or gas flow paths for feeding a supply of gas to the respective unit cells are formed inside the cell laminate in the form of through apertures that pass through the constituents of the cell laminate such as separators. In the external manifold type, on the other hand, the manifolds are arranged outside the cell laminate.
In the prior art method wherein a solution obtained by dissolving a resin in a solvent is applied and dried or a reactive resin is applied and solidified in order to form the gas seal portion that covers the side face of the cell laminate, however, there is a problem that the sufficient gas sealing property cannot be given.
When the manifolds, which connects with gas inlets and outlets, are provided, the significant unevenness on the surface of the gas seal formed by the resin makes it difficult to ensure the favorable gas sealing property at a portion where the side face of the cell laminate is in contact with the manifold.
For example, there is a method which casts a thermosetting resin such as an epoxy resin into a cast mold which envelopes the cell laminate to integrally mold, but solidification of the resin takes time to bring about poor productivity.
Any of the above method has another problem that the gas inlets and outlets are closed by the air-tight non-conductive material.
Around the electrodes, sealing portions like gaskets are disposed and sandwiched between a pair of separator plates in order to prevent the reaction gases fed to the cathode and the anode from being leaked. The prior art technique arranges hard gaskets composed of, for example, a fluorocarbon resin, around the peripheral portion of the electrodes and subsequently places a pair of separator plates across the gaskets and, therefore, there needs the accurate adjustment of the thickness of the electrodes and the gaskets.
In the case where the gaskets have rubber-like elasticity, however, the strict size accuracy is not required, but the function of the gaskets can be attained by ascertain level of adjustment of the thickness. The properties required for the gaskets thus include acid resistance and the rubber-like elasticity. Although having the poorer acid resistance than the fluorocarbon resin, ethylene-propylene-diene rubber (EPDM) having elasticity is sometimes used for the material of the gaskets.
The separator plates are directly in contact with the electrodes and are thereby required to have high gas tightness and electrical conductivity, as well as the acid resistance. When the air is used as the oxidant gas, it is required to enhance the flow rate of the air supplied to the cathode and to efficiently remove liquid water or water vapor evolved at the cathode. A complicated structure generally called the serpentine-type as shown in FIG. 5 is typically applied for the gas flow path structure in the separator plate. The separator plate is obtained by cutting a carbon material such as a dense carbon plate having gas tightness, a carbon plate impregnated with a resin, or glassy carbon to a desired shape and forming grooves for gas flow paths. In another example, the separator may be obtained by processing and plating a corrosion-resistant alloy plate with a noble metal on demand.
Also, on demand, the carbon material or the corrosion-resistant metal material may be used only for the portions that are in contact with the electrodes and require the sufficient electrical conductivity. Further, there has been an attempt that the separator plates of a resin-containing composite material may be used for the peripheral portions such as manifolds, which do not require the electrical conductivity. Also, there is suggested that a resin is mixed with carbon powder or metal power and press-molds or injection-molds.
However, the fluorocarbon material is employed for the sealing portions like gaskets, there is a problem of high cost. The fluorocarbon material is generally a very hard resin and requires an extremely large load to clamp the gaskets and sufficiently seal the flow of gas or cooling water. Therefore, there are attempts to use the porous fluorocarbon material or to apply the fluorocarbon paste on the separator plates, which are used in dry or half dry state. The porous fluorocarbon material is, however, expensive. In addition, there is a problem that the sufficient sealing properties cannot be attained when the load applied for clamping is not as large as the level that damages the porosity.
The fluorocarbon paste used for the sealing portions still has high material cost. When dried and cured, the hardness thereof makes it difficult to regulate the thickness at the time of the application. The rubber material like EPDM does not have so high acid resistance as the fluorocarbon resin and is thus not suitable for the long-term use. The general EPDM has thermoplasticity and is deformed with time at the cell-driving temperature as of 80xc2x0 C. In some cases, there is a problem that the deformation blocks-the gas flow path and lowers the supply of the fuel.
With respect to the material for the separator plates, in the case where the dense carbon plate having gas tightness or glassy carbon is employed for the separator plates, the cutting work is required to form the gas flow paths. This is undesirable from the viewpoints of mass production and manufacturing cost. In the case where the carbon plate impregnated with a resin is used for the separator plates, impregnation of the resin after formation of gas flow paths causes warpage of the carbon plate because of little elasticity of the resin. Post treatment including the work of cutting the gas flow paths should accordingly be required after impregnation of the resin. When a phenol resin or a silicone resin is used as the impregnating material, the sufficient acid resistance cannot be attained. In the case where the corrosion-resistant alloy or the material plated with a noble metal is used, the cutting work is required to form the serpentine flow path structure.
In the case where the mixture of a resin and carbon powder or metal powder is press-molded or injection-molded to separator plates, the resin itself is required to have acid resistance. The fluorocarbon resin or another hard resin material has low fluidity and difficulty in molding. The resin having poor fluidity allows only a low content of the resin in the mixture. In this case, post treatment, for example, impregnating the portions that require the gas tightness with the resin, is required after the molding. This results in the complicated structure.
The object of the present invention is thus to provide a polymer electrolyte fuel cell stack having excellent durability of seals. The object of the present invention is also to provide a method of manufacturing such a polymer electrolyte fuel cell stack with a high productivity.
The present invention relates to a polymer electrolyte fuel cell stack (hereinafter simply referred to as PEFC) comprising a cell laminate having a plurality of unit cells, which are laid one upon another, and each of which includes a polymer electrolyte membrane, a pair of electrodes that are arranged across the polymer electrolyte membrane and respectively have a catalytic reaction layer, a separator having means for feeding a supply of fuel gas containing hydrogen to one of the electrodes, and another separator having means for feeding a supply of oxidant gas to the other of the electrodes, wherein a sealing portion is disposed at least in the vicinity of each electrode.
It is preferable to have the sealing portion over a whole side face of the unit cells. It is also preferable that the separator has cooling means with a sealing portion. It is preferable to have the sealing portion around each electrode or in a space formed in the vicinity of each electrode between the separators. It is preferable to have a humidifying unit that enables heat exchange between a flow of cooling water discharged from the polymer electrolyte fuel cell stack and a flow of fuel gas fed to the polymer electrolyte fuel cell stack and simultaneously carries out heating and humidifying, the humidifying unit having a sealing portion.
Also, it is preferable that the sealing portion is constituted by a polymer compound expressed by Formula (1) given below: 
wherein R1 and R2 are non-functional end groups; Xi and Yi are independently polymerizable functional groups and form crosslinking points after polymerization; m is an integer of not less than 1 and represents a number of repeated isobutylene oligomer units; and i is an integer of not less than 1 and represents the degree of polymerization.
Further, it is preferable that the sealing portion is constituted by a mixture of the polymer compound expressed by Formula (1) given above and an electron-conductive material.
In addition, it is preferable that the separator is constituted by a carbon material or a metal material and a polymer compound expressed by Formula (1) given below: 
wherein R1 and R2 are non-functional end groups; Xi and Yi are independently polymerizable functional groups and form crosslinking points after polymerization; m is an integer of not less than 1 and represents a number of repeated isobutylene oligomer units; and i is an integer of not less than 1 and represents the degree of polymerization.
Also, it is preferable that the polymerizable functional groups Xi and Yi in Formula (1) given above are independently selected among the group consisting of allyl group, acryloyl group, methacryloyl group, isocyanate group, and epoxy group.
It is also preferable that the number of repeated isobutylene oligomer units m in Formula (1) given above ranges from 56 to 72.
It is further preferable that the degree of polymerization i of the polymer compound in Formula (1) given above is not less than 8000.
Also, it is preferable that at least portion of the sealing portion is formed by injection molding a non-conductive gas-tight material.
Further, it is preferable that at least portion of the sealing portion consists of plural layers, and an outer-most layer thereof is formed by injection molding.
Also, it is preferable that a manifold for feeding the supply of fuel gas or the supply of oxidant gas to each of the electrodes is disposed on a side face of the cell laminate.
It is also preferable that the sealing portion has a three-layered structure, where a heat-resistant hard resin layer is interposed between a pair of elastic resin layers.
It is preferable that the heat-resistant hard resin layer is constituted by a polyethylene terephthalate resin and the elastic resin layer is constituted by a polymer compound expressed by Formula (1) given below: 
wherein R1 and R2 are non-functional end groups; Xi and Yi are independently polymerizable functional groups and form crosslinking points after polymerization; m is an integer of not less than 1 and represents a number of repeated isobutylene oligomer units; and i is an integer of not less than 1 and represents the degree of polymerization.